1. Field of the Invention
The invention relates to a process for the preparation of thermally stable vinyl chloride copolymers, processes for the preparation thereof, and to the use thereof, in particular in coating resin compositions.
2. Background Art
A major problem with the use of vinyl chloride copolymers is the insufficient stability of solutions of these polymers in organic solvents, in particular in ketones. In warm regions having an ambient temperature of more than 30° C., a yellow or brown discoloration of the originally colorless solutions occurs after storage of such solutions for only a few days. Also problematic is the storage of pulverulent resins based on vinyl chloride copolymers at elevated temperatures. Here, discoloration of the powder may occur after a storage time of only a few months, subsequently leading to discoloration of solutions of these powders.
The lack of thermal stability of these copolymers is said to be a reason for the tendency of solutions of vinyl chloride copolymers to become discolored. Other adverse phenomena are corrosion phenomena in the form of rust formation on metal parts of machines and storage containers which come into contact with solutions of vinyl chloride copolymers.
EP-B 358067 proposes the use of vinyl chloride copolymers comprising from 1 to 20% by weight of glycidyl methacrylate and from 12 to 40% by weight of further comonomers for improving the resistance to yellowing. EP-B 219097 discloses that the thermal stability can be improved by copolymerizing vinyl chloride with OH-functional acrylates, epoxide-containing comonomers and acid-functional comonomers. For the epoxide-containing comonomers, such as glycidyl methacrylate, it is recommended to use these in an amount of from 1 to 12% by weight. Similar copolymers form the subject of EP-B 264196, in which a content of less than 0.5% by weight of epoxide groups, corresponding to about 1.7% by weight of glycidyl methacrylate, is described as being insufficient for improving the thermal stability.